Sound translating device



June 29, 1937. H, R WARNKE 2,085,7'21

- SOUND TRANSLATING DEVICE Filed Oct. 29, .1932

Patented June 29, 1937 SOUND 'rnaNsLa'rrNG DEVICE Herbert R. Warnke,Camden, N. I., assigner to Radio`Corporation of America, New York, N.Y., a corporation of Delaware Application october 29, 1932, serial No.000,171

10 ciaims. (ci. 17a-115.5)

My invention relates to sound translating devices and, moreparticularly, to loudspeakers of the movable coil or dynamic type havinga permanent magnet field and a large direct-acting conical diaphragm.

In the construction of dynamic loudspeakers having a permanent magneteldwith which I have been familianlarge unwieldly eld yokes, usuallycomprising a multiplicity of bars, have been employed and theconstruction has been lacking in the simplicity, compactness, andruggedness characteristic of certain makes of dynamic speakers of theelectromagnetic type.

In these permanent magnet field dynamic speakers, it has been thepractice heretofore to secure the parts Vtogether by means of bolts,clamps, or the like. In such structures, the field magnet has usuallybeen of extremely hard chrome-steel and it has been necessary to softenthe metal by special heat treatment prior to forming a large hole in theeld yoke for the purpose of securing the center pole piece thereto. Ithas been found, however, that this operation weakens the materialappreciably and, upon subsequent hardening of the metal, considerabletrouble has been experienced because of a tendency of the yoke to crackand deform from its previous shape. y

InV other respects, prior constructions of the above character lackedcertain desirable features, particularly in regard to cost ofmanufacture and magnetic efciency considered from the standpoint ofmagnetomotive force in relation to weight and bulk of material used inthe eld structure.

Itis, accordingly, an object of my invention. to provide a dynamicspeaker of the type having a permanent magnet field and a diaphragm ofthe direct acting type having appreciable active area andY depth, thatshall be compact in form and particularly adapted for use in automobileinstallations where space is quite limited.

A further object of my invention is to provide a permanent magnet fieldstructure for a dynamic loudspeaker, having considerably increasedmagnetic eciency, with a minimum of Weight and bulk of magnetic materialused.

A still further object of my invention is to provide a permanent magnetfield structure for a loudspeaker of the dynamic type, whereby themanufacture and assembly thereof may be facilitated and the operationand adjustments incident thereto requiring great skill, may besimplified,

thereby resulting in a device that may be manufactured in quantitieswith a high degree of` uniformity and low cost.

The invention will, however, be better understood when taken inconnection with the accompanying drawing, and its scope will be pointedvout in the appended claims. Y

Referring to the drawing, Fig, 1 is a rear view in perspective and on areduced scale,` of adynamic loudspeaker embodying my invention; Y

Fig. 2 is aside YView in section, slightlyenlarged of the deviceshownAin Fig. 1and includingin addition, a housing structure; and f Y Fig. 3is a side View of a modification of the eld yoke structure of the deviceshown in Figs. 1 and 2 and embodying my invention.`

According to one phase of my'invention, a permanent magnet eld structurefor a cone type dynamic speaker. is constructed, preferably of a singleyoke bar, to produce a magnetic field of great strength by means of afield yoke structure u so formed and arranged with respect Vto the backof the'conediaphragm that a permanent magnet yoke oi' relativelygreatlength isrpossible ,with--V outany substantial waste of availablespace from front to back `of the speaker, thereby producing a compact,Simple, and highly eiflcient structure. I am aware of the fact thatcompact dynamic cone speaker constructions have been made in the past,e. g. by placing the dynamic eld structure entirely or partially withintheconcave side of the cone diaphragm, e. g. in the manner disclosed inapplication of A. Ringel, Serial No. 515,190, `filed February 12, `1931`and assigned to RCA Victor Company, Inc., Camden, New Jersey.

Referring to Figs. 1 and 2, the sound translating device shown is adynamic loudspeaker of the permanent magnet eld type, representing-apreferred embodiment of my invention, and in-l cludes a permanent magnetin the form of-a wide U-shaped yoke member l of chrome-steel inconjunction with a relatively short inner pole piece 3 of ferro-magneticmaterial, e.. g. cold rolled steel, secured at its lower end tothe yoke,pref-k erably by electric arc llet welding. The lower end of the po-lepiece 3iis preferably enlarged or hanged to-form a flux plate 4 whichprovides a better support and gives aV larger volume of ma-V terial aswell as a greater area of magnetic contact to thereby reduce thereluctance. One of a pair of diametrically opposite lllet welds is shownat l! along a lower edge of the flange. or the ux plate d. The upper endof thecenter pole'pieceterminates at a point appreciably below the levelof the ends-of the U yoke member I.

An inwardly bent yoke plate 5, which bridges the yoke magnet I, is madeof ferro-magnetic material, e. g. soft iron of high permeability, andserves as a pole piece for the permanent magnet eld yoke I. The yokeplate 5 is provided with a central opening 'I in the inwardly directedportion for cooperating with the center pole piece 3, and is secured atits ends, preferably on opposite sides thereof, to the outer ends of theU-shaped yoke, preferably by electric arc llet welding, as shown at 9 inFig. l. During welding, the parts are held in such manner that thecircular opening is in concentric relation with the pole piece, therebyforming an annular air gap. The abutting faces of the yoke plate 5 andthe ends of the U, as well as the bottom face of the flange I on theinner pole piece and the face of the yoke in Contact therewith, are allsurface-ground prior to assembly and welding. 'I'he parts are assembledby means of a special form that is adapted to hold the parts inassembled and spaced relation during the Welding process, the novelstructure being particularly adapted for assembly by the foregoingoperations. A voice coil and form I3 attached to the small end of afrusto conicaldiaphragm I5 is supported for vibration in the air gap, aresilient spider I6 being secured to the cone and attached to themagnetic structure, preferably to the free end of the inner pole piecefor centering the coil in said gap.

For the purpose of supporting the base of the diaphragm, a bracketstructure is provided therefor by securing, as by welding in similarmanner, an annular metallic ring II preferably to the outer ends of theyoke plate 5. A treated fabric structure I9, for flexibly supporting thebase of the cone, and attached thereto, is firmly clamped between thisring l1 and an adjacent metallic ring 2l adapted to be held together, asby rivets 2,3.

The ends of the permanent magnet field yoke I are preferably joined tothe ends of the yoke plate 5 adjacent to or substantially in the sameplane with the base of the cone. The yoke plate 5, which constitutespart of the cone supporting bracket structure, is inwardly bentsubstantially in conformity with the contour of the conical diaphragmand into operative association with the shortened inner pole piece.While I have shown, as my preferred embodiment, a bracket structureformed by securing the ring II directly to the ends of the yoke plate 5,for supporting the cone, it should be obvious that a separate bracketstructure not associated with the ends of the yoke plate could beprovided for the diaphragm. If desired, the ring l1 and yoke plate 5could be integrally formed out of a single plate.

The foregoing field structure is especially adapted to be magnetizedafter the assembly has been completed. It is particularly necessary tomagnetize after assembly because of the fact that, if the field yoke ismagnetized beforehand, as has been previously, of necessity, done inmany cases, by reason of the construction, the strong magnetic fieldtends to attract stray particles of iron to the air gap, and it has beenfound quite dicult and impractical to free the gap of such particlesafter they are once there.

While the yoke bar I appears to be one magnet, and is so structurally,magnetically it is two magnets in parallel, each being constituted bythe portion from the Amiddle of the bar at the inner pole piece to anouter end of the U. The ends of the U are of one and the same polarity,While the middle portion is of the opposite polarity. This arrangementhas resulted in a highly eifrcient structure, giving a strong intensemagnetic field, and it has been particularly well adapted to availablematerials. It has been found, for example, practicable to harden magnetchromesteel bars of a thickness of only about 5/8 inch, the thickness ofthe yoke bar used in speakers built in accordance with my invention. -Inorder to obtain the necessary flux at the air gap for proper operation,it has been found necessary, when using magnetic material of the abovenature, to employ a plurality of bars physically, as Well asmagnetically, in parallel. In one construction with which I am familiar,a pair of bent magnet bars were used in parallel in a superposedco-extensive relation, with their opposite ends attached to the two polepieces respectively. The resulting structure was quite unsymmetrical andproved difficult to manufacture.

As shown in Fig. 2, the speaker assembly is mounted in a sheet metalhousing 21, preferably of non-magnetic material, such as aluminum, bymeans of bolts 29 that pass through the annular rings ll and 2l on theassembly and an inwardly turned flange SI on the housing. The rear endof the metal housing is open except for a cover-r ing of fabric 33 inorder to prevent the entrance of foreign particles into the assemblywhile not interfering 'with the free passage of sound Waves. The frontof the housing is likewise covered with a protecting layer of fabric315. The housing is supported at its rear by means of a bracket 35adapted to be attached in automobiles to a surface of substantial area,as to the dashboard 31, and is preferably spaced therefrom, as shown,for desired acoustic effects.

Referring to Fig. 3, I have shown a modified structure that makespossible a still more efl'lcient arrangement wherein the permanentmagnet field yoke is lengthened by bowing outwardly the sides 2 of theU, thereby giving a higher magnetomotive force. The ends of the Uextend` inwardly into engagement with the yoke plate 5 at points spaceda short distance from the ends of said plate. This arrangement permits ashortening of the magnetic path through the iron yoke plate in themanner shown, which is desirable. The yoke plate 5, preferablydropforged, is preferably provided with offset portions 43havingparallel ground faces for contacting with the angularly faced endsof the field yoke at 4I. The angular face formed on the ends of theinwardly bent U at lll results in a greater area of contact between theground surfaces, and thereby appreciably lowers the magnetomotive forceloss at the juncture between the pole portions.

Several features of my abov'e construction have been shown! and/ordescribed, but not claimed, in the application of Creager et al,

Serial No. 589,824, filed January 30, 1932, assigned to the sameassignee, and now U. S. Patent No. 2,027,473.

By means of my above described novel arrangements, the eiciency of thefield structure of the permanent magnet dynamic device has been greatlyincreased over prior constructions by reason of. the construction thatpermits the employment of a yoke of relatively great length andcross-section. At the same time, a compact arrangement, particularly inthe direction from front to back, has been effected by reason` of thefact of the novel combination or association of the magnetic field andconical diaphragm structure. Furthermore, my novel structure has madepossible the production or" a relatively inexpensive construction thatis particularly adapted to large quantity production with a high degreeof uniformity in the output.

I claim as my invention:

l. In a permanent magnet eld system for a sound translating device ofthe dynamic type, a U-shape bar of extremely hard ferro magneticmaterial, the sides of said U being inwardly bent, an inner pole pieceattached at one end to the back of said U-shape yoke, and an inwardlydirected yoke plate adapted to form a magnetic circuit and an air gapbetween the ends of the said U and the other end of. said inner polepiece, said yoke plate having offset portions adjacent its ends forattachment to the endsof said U-shape yoke.

2. In a permanent magnet loud speaker of the electrodynamic type, thecombination of a Wide permanent magnet of rectangular cross section, acylindrical pole piece secured to one side of and extending at a rightangle to the intermediate portion of said magnet, and a ux platedisposed between said pole piece and the magnet, said flux plateextending at both sides of the pole piece to the edges of said magnet toprovide a low reluctance magnetic flux path between the magnet and saidpole piece.

3.Ir1 a permanent magnet loud speaker of the electrodynamic type, thecombination of a wide permanent magnet of rectangular cross sectionhaving its ends of the same polarity and its intermediate area ofanother polarity, a round pole piece secured to said magnet at itsintermediate area and projecting outwardly therefrom, and a ux platedisposed between said pole piece land said magnet, said ux plateextending from the pole piece to the edges of the magnetV wherebysubstantially all of the magnetic flux at the intermediate area of theVmagnet will be directed into the pole piece.

4. In an electrodynamic loud speaker, the com- V bination or". aU-shaped permanent magnet of rectangular cross section having ends ofthe same polarity and an intermediateA area of another polarity, a roundpole piece of magnetic material secured upon said U-shaped magnet at itsintermediate area and projecting between and parallel with its ends, abridging member of soft iron welded to the ends of said U-shaped memberhaving an aperture into which the end of said round pole piece projectsto form an annular air .gap of magnetic iiux, and means between saidround pole piece and the intermediate area of the magnet for directingthe magnetic flux at the intermediate area of the magnet into said polepiece,

5. A permanent magnet field structure for an electrodynamic loudspeaker, comprising a permanent bar magnet of rectangular cross sectionhaving ends of the sam-e polarity disposed in spaced parallel relationand its middle po-rtion of another polarity, an iron pole piece securedacross the ends of said bar magnet and having 'its middle portiondisposed inwardly between the ends of the magnet in spaced parallelrelation with the middle portion of the magnet, a cylindrical iron polepiece mounted upon and projecting from the middle portion of the magnetfor completing a magnetic circuit between the first pole piece and themiddle portion of the magnet, and means engaging the full width of saidbar magnet at its middle portion for directing the magnetic flux of themagnet into said cylindrical pole piece.

6. A permanent magnet eld structure for an electrodynamic loud speaker,comprising a permanent :magnet of rectangular cross section having itsends of the same polarity and its intermediate portion of anotherpolarity, the ends of said permanent magnet being disposed at a rightangle to the intermediate portion of the magnet, a short pole piece ofround cross section secured between the ends of said permanent magnetand projecting in the samedi-Y rection as said ends, said short polepiece having a length less than that of theends of the permanentmagnet,V a magnetic flux conducting member secured across the ends ofsaid v72 In an electrodynamicloud speake'rgfthe combination of aU-shaped member of magnetized steel having ends of the same polarity, apole piece of magnetic material secured upon said U-shaped memberintermediate and parallel with the ends thereof, f a bridging member Yofsoft i iron secured across the ends of said U-shaped member having anaperture into which the end of said pole piece projects to form anannular air gap, and a flux directing member of magnetic materialintrposed between saidpole piece'and the intermediate portion Yof saidU-shaped member.

8. In an electrodynamic sound reproducing device of the characteremploying a permanent magnet for maintainingia magnetic flux Vin its airgap, the combination of a. U-shaped member of rectangular cross sectionformed of. magnetized'steel, an outer pole piece member having anaperture therein secured upon said U-shaped member at a point of onepolarity, an inner pole piece member of round cross section having oneend disposed within the aperture of said outer pole piece member to forman air gap and the other end adjacent a point of another polarity onsaid U-shaped member, and means at the latter end of said inner polepiece engaging the full width of said U-shaped member for providing amagnetic path of low reluctance between said inner pole piece and said-U-shaped member.

9. The invention set forth in claim 2 characterized in that saidcylindrical pole-piece and said flux plate are constituted by anintegral structure and characterized further in that said iiux plate isWelded to said magnet.

10. The invention set forth in claim 6 characterized by the addition ofa substantially conical diaphragm provided with a voice coil, said voicecoil being disposed in said aperture andlabout the adjacent end vof saidshort pole-piece, and characterized further in that said magnetic uxconducting member is disposed inwardly substantially in conformity withthe contour of. said 70 diaphragm into cooperative association with saidshort pole-piece.

HERBERT R. WARNKE.

